Polyimides comprise polymers characterized by excellent thermal stability, solvent resistance and high glass transition temperatures (Tg). For example, in structural applications, the fiber-reinforced high-temperature polyimide matrix composites offer significant advantages over metals because of their low density and high specific strength. These composites are particularly attractive for use in aerospace systems, e.g. aircraft engines, airframe, missiles, and rockets, where weight is critical. Weight reduction has substantial benefits in terms of fuel savings, increased cargo load, or increased speed and maneuverability. The durability and reliability of the type of polymers used in aerospace components is a critical concern. The polymer requirements for these applications include high glass transition temperatures, (Tg), high thermo-oxidative stability and good mechanical properties over a wide range of temperatures. In general, the stability of most polymers are limited to 50° C. lower than their glass transition temperature (Tg).
The prior art describes polyimides end-capped with diaryl substituted acetylene by dissolving the monomers, i.e. the dianhydride, diamine and the endcap in organic solvents such as N,N-dimethylformamide or N-methyl-2pyrrolidinone (NMP) and subsequently imidizing the oligomers through heating. The oligomers are isolated by precipitation of the imide solution in water. In the 48th SAMPE Symposium, 1076-1086 (2003), the imide oligomer was prepared by dissolving 2,3,3′,4′-biphenyl dianhydride (a-BPDA) together with two diamines and 4-phenylethynylphthalic anhydride in NMP and subsequently imidizing the oligomer by precipitating the slurry in water. The imide oligomers based on a-BPDA, were cured at 371° C. for 1 hour to form polyimides having Tg's equal to 330° C.
The problem that arises from this process is that NMP or any other organic solvent is very difficult to remove from the oligomers. Even when the imide oligomer was recovered by precipitation from the water, traces of NMP were still present. The presence of trace amounts of NMP or any other solvent during the curing or resin transfer molding process (RTM), for example, often creates voids or causes delamination in the final composites. To remove the last trace of solvent, extensive drying under vacuum or passing nitrogen gas in a convection oven are often needed. These extensive measures generally cost additional time and labor which is not very cost effective.
One of the objects of this invention is to produce low-melt viscosity imide oligomers with a reactive endcap to form thermoset polyimide resins by a melt process free of any solvent in order to reduce the manufacturing cost due to the solvent removal process. The oligomers can be cured via the reactive endcap to form thermoset polyimide resins. Accordingly, through the selection of specific diamines together with certain dianhydrides such as 2,3,3′,4′-biphenyl dianhydride (a-BPDA) and phenylethynylphthalic anyhdride as the endcap, this invention produces novel imide oligomers capable of achieving low-melt viscosities of 1-60 poises. Upon curing these oligomers at 371° C. (700° F.), the imide oligomers yield thermoset polyimide resins having high glass transition temperatures (Tg) equal to or greater than 330° C. These oligomers are adaptable to the RTM, VARTM or resin infusion processes and also can be formulated into adhesives capable of high temperature applications between 288-343° C. (550-650° F.).
The significant advantages of melt processing in accordance with the present invention includes processing the oligomers without the use of solvent such that costly solvent recycling is unnecessary and can be eliminated. High thermal stability is not only essential for processing the melt at temperature greater than or equal to 350° C., but is required also for polyimides for use in various other high temperature applications.
Moreover, the thermosetting polyimides of this invention are easier to process in comparison to other polymers such as the thermoplastic resins in that they have a lower molecular weight and lower viscosities. The thermosetting polyimides have superior processability in addition to their high temperature capabilities which make them more attractive for use as high performance matrix resins in preparing lightweight, fiber-reinforced polymer matrix composites. These polymer matrix composites are finding increased use in the electronics, automobile and aerospace industries.